Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy the requirements of the customers, resulting in a large loss of profits. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708 is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a printhead, and an imaging drum), and thermal print media and dye donor material exit transports.
The scanning subsystem or write engine of the lathe bed scanning type comprises a mechanism that provides the mechanical actuators, for imaging drum positioning and motion control, to facilitate placement, loading onto, and removal of thermal print media and dye donor material from the imaging drum. The scanning subsystem or write engine provides the scanning function by retaining the thermal print media and dye donor material on the rotating imaging drum, which generates a once per revolution timing signal to data path electronics as a clock signal, while the translation drive traverses the translation stage member and printhead axially along the imaging drum in a coordinated motion with the imaging drum rotating past the printhead. This is done with positional accuracy maintained, to allow precise control of the placement of each pixel, in order to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the imaging drum and its rotational drive. The translation drive with the translation stage member and printhead are supported by two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the vacuum imaging drum and a lead screw. Consequently, they are parallel to each other therein forming a plane, along with the imaging drum and lead screw. The translation bearing rods are, in turn, supported by outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned therebetween, for permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are sufficiently rigid for this application, so as not to sag or distort between the mounting points at their ends. They are arranged to be as exactly parallel as is possible with the axis of the imaging drum. The front translation bearing rod is arranged to locate the axis of the printhead precisely on the axis of the imaging drum, with the axis of the printhead located perpendicular, vertical, and horizontal to the axis of the imaging drum. The translation stage member front bearing is arranged to form an inverted "V" and provides only that constraint to the translation stage member. The translation stage member with the printhead mounted on the translation stage member, is held in place by only its own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod. This is done so as to provide no over constraint of the translation stage member which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to the translation drive or printhead during the writing process causing unacceptable artifacts in the intended image. This is accomplished by the rear bearing which engages the rear translation bearing rod only on a diametrically opposite side of the translation bearing rod on a line perpendicular to a line connecting the centerlines of the front and rear translation bearing rods.
The translation drive is for permitting relative movement of the printhead by synchronizing the motion of the printhead and stage assembly such that the required movement is made smoothly and evenly throughout each rotation of the drum. A clock signal generated by a drum encoder provides the necessary reference signal accurately indicating the position of the drum. This coordinated motion results in the printhead tracing out a helical pattern around the periphery of the drum. The above mentioned motion is accomplished by means of a DC servo motor and encoder which rotates a lead screw that is typically, aligned parallel with the axis of the imaging drum. The printhead is placed on the translation stage member in a "V" shaped groove, which is formed in the translation stage member, which is in precise positional relationship to the bearings for the front translation stage member supported by the front and rear translation bearing rods. The translation bearing rods are positioned parallel to the imaging drum, so that it automatically adopts the preferred orientation with respect to the surface of the imaging drum. The printhead is selectively locatable with respect to the translation stage member, thus it is positioned with respect to the imaging drum surface. By adjusting the distance between the printhead and the drum surface, as well as the angular position of the printhead about its axis using adjustment screws, an accurate means of adjustment for the printhead is provided. Extension springs provide the load against these two adjustment means.
The translation stage member and printhead are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and printhead along the threaded shaft as the lead screw is rotated. The lateral directional movement of the printhead is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
The printhead includes a plurality of laser diodes which are coupled to the print-head by fiber optic cables which can be individually modulated to supply energy to selected areas of the thermal print media in accordance with an information signal. The printhead of the image processing apparatus includes a plurality of optical fibers coupled to the laser diodes at one end and the other end to a fiber optic array within the printhead. The printhead is movable relative to the longitudinal axis of the imaging drum. The dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead and thus to the dye donor material is converted to thermal energy in the dye donor material.
The design of scanning subsystems for image processing apparatuses presents strict constraints. Chief among these are the following:
There is a requirement for precision timing so that the imaged dots are written in the intended location on the receiving medium, with acceptable error tolerances typically on the order of a few microns; PA1 One must compensate for some irregularity in imaging drum rotational speed, so that the translation drive may need to be dynamically sped-up or slowed down to provide the required printhead position; and PA1 One must be able to adapt to writing using a variable number of channels. As noted in U.S. Pat. No. 5,329,297, there can be specific halftone screen patterns that cause inherent problems when imaged with specific number of channels (swath width) due to "beat frequency" problems.
Conventional solutions to the above-listed design constraints are known to be relatively expensive and inflexible. The use of a servo motor mechanism, as described above for the device disclosed in U.S. Pat. No. 5,268,708, is workable, but is expensive since a servo system requires a precision feedback loop. To adapt to imaging using a variable number of channels, a printer controller subsystem is used to adjust both the imaging drum speed and corresponding servo motor speed for the translation subsystem, as is disclosed in U.S. Pat. No. 5,329,297.
A less expensive alternative to the servo subsystem described above is to use a stepper motor for the translation system. This approach is less expensive and allows the translation subsystem to be operated "open-loop" (that is, without requiring feedback components for motor timing). However, the stepper motor must be controlled so that it repeatably provides the precise speed needed to write the image using a variable number of channels. The stepper motor must also be controlled with precise timing, so that printhead travel speed adjusts for small changes in imaging drum speed and thus maintains positional accuracy.